What are the rules for getting a 1987 unregistered USA RV from UK to ROI?

There aren't any rules as such - but if you need to drive it from somewhere in UK to a ferry port that connects with ROI you will need, at the very least, insurance which is valid for UK. Many UK insurers will insure an import on its VIN number to let you get a vehicle home and then you do the registration process (Pay import duty, get roadworthiness tests, UK reg etc, etc) As your final destination will be ROI you need to talk to an Irish insurance company and see if they can do this. Otherwise its on the back of a transporter to ferry port and then its up to you when it drives off.

You'll have to pay the extortionate Vehicle Registration Tax (VRT) once You get to the ROI.